This invention relates to an apparatus which will give an audio or visual indication to the operator of a vehicle of an abnormal pressure condition in a tire on the vehicle. More particularly, the invention relates to such an apparatus which is fully testable by the operator to assure that the components thereof are properly functioning.
An abnormal or undesirable pressure condition in a pneumatic tire, if undetected particularly in situations involving long trips or the like, can be quite detrimental to the tire and could also represent a hazard to the safe operation of the vehicle. A number of alarm circuits have been designed to detect abnormal pressure conditions. One such device detects an abnormal pressure condition by counting tire revolutions and comparing the same with those of a properly inflated tire. However, these devices are not satisfactorily workable with a radial tire, where pressure tolerance is highly important, because there is little tire deflection under slightly abnormal pressure conditions in the radial tire. Similarly, in the newly developed cast tire, there is very little deflection change related to pressure.
Other devices utilize radio circuits or ultrasonic systems to warn the operator of a vehicle of the undesirable pressure conditions. Such devices are quite susceptible, however, to spurious activation from other transmissions or total failure due to transmission blockage by the metallic portions of the vehicle body.
More recently a highly sophisticated system has been developed which utilizes a system of mirrors or the like to transmit light signals upon each revolution of the tire. Absence of such a signal, indicative of an abnormal tire pressure condition, triggers an alarm circuit. Such a system is described in pending application Ser. No. 581,392 filed May 27, 1975 and owned by the assignee of this application. Although it is possible that through a buildup of dirt or the like in the optical system discussed in that application, a malfunction could occur, that system is entirely workable under most conditions. However, because of the sophistication of that system, it is testable by the vehicle operator only when the vehicle is stopped or travelling at low speeds and, in addition, is more expensive to install and maintain.